Many aspects to lens performance
There are many aspects to lens performance, some subtle, and some not so. One aspect is clearly evident in the image below (distortion).
When I review lenses, I look for all sorts of things, and provide real insight into what a lens actually does with real images—this sort of knowledge takes quite some effort to acquire even when you’re aware of all the possibilities. No doubt I miss a few things, but I don’t miss very much, and I share it all in my full lens reviews such as Zeiss ZF Lenses. That’s why even if you’re only a little interested in a Zeiss ZF lens you’ll find the review tremendously helpful in understanding lenses in general.
Nikon D3 + Zeiss ZF 18/3.5 Distagon @ f/8